Electrochemical capacitors are a class of high rate energy storage devices which use electrolytes and electrodes of various kinds in a system similar to that of conventional batteries. Electrochemical capacitors like batteries are essentially energy storage devices. However unlike batteries, capacitors rely on charge accumulation at the electrolyte/electrode interface to store energy. Charge storage in electrochemical capacitors therefore, is a surface phenomena. Conversely, charge storage in batteries is a bulk phenomena occurring within the bulk of the electrode material.
Electrochemical capacitors can generally be divided into one of two subcategories. Double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and pseudocapacitor devices in which charge transfer between the electrolyte and the electrode occurs over a wide potential range, and is the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte. These types of electrochemical capacitors are currently being developed for high pulse power applications.
Most of the known electrochemical capacitor active materials are based on metallic elements such as platinum, iridium, ruthenium, or cobalt. These materials are generally quite expensive and pose a significant hurdle to the widespread commercialization of this technology. Moreover, to the extent that these devices have been fabricated, they have heretofore, relied upon traditional battery technology when approaching the question of appropriate electrolytes to be used in connection with such devices. Accordingly, electrolytes used in these devices have generally been conventional aqueous electrolytes such as potassium hydroxide (KOH). Unfortunately, electrochemical capacitor devices using KOH electrolytes have failed to satisfy commercial needs for various reasons including for example, poor cycle life and an inability to achieve desired electrochemical performance characteristics.
In addition to the performance problems described above, electrochemical capacitor devices have also suffered from problems associated with the manufacture and packaging of such devices. It is the nature of electrochemical capacitors to require relatively small packages which develop high pulse power spikes. Prior art methods of assembling such devices however substantially increased the thickness of the device, as well as the complexity of the manufacturing process. Increased complexity resulted in manufacturing defects which caused yield losses. Moreover, as the package became thicker due to processing, the introduction of electrode equivalence series resistance (ESR) reduced the efficiencies of the devices fabricated.
Accordingly, there exists a need to provide a new process for manufacturing electrochemical capacitor devices. This process should emphasize ease and convenience of manufacturing while providing a thin profile device so as to reduce ESR.